1. Field of the Invention
This invention relates to a method for producing silicone resins. More specifically, the present invention relates to a method for producing silicone resins composed of difunctional siloxane units (D units) and trifunctional siloxane units (T units). The resins exhibit a low molecular weight dispersivity and a controlled molecular weight.
2. Background Information
Silicone resins that are liquid at room temperature or that have relatively low softening points are employed as electrically insulating varnishes, heat-resistant coatings, and sealants for semiconductor elements. These silicone resins are classified according to the particular combination of siloxane units that make up the resin. Examples of such resins are DT type resins composed of difunctional siloxane units (D unit) and trifunctional siloxane units (T unit), MQ type silicone resins composed of monofunctional siloxane units (M unit) and tetrafunctional siloxane units (Q unit), and DTQ silicone resins composed of difunctional siloxane units (D unit), trifunctional siloxane units (T unit), and tetrafunctional siloxane units (Q unit). Among these silicone resins the DT type are characterized by ease of formation of the cured film by crosslinking and by the excellent mechanical properties of the cured film.
The DT type of silicone resins are prepared by cohydrolysis of a organotrihalosilane and a diorganodihalosilane or the corresponding organotrialkoxysilane and diorganodialkoxysilane in water. The resultant hydrolysis product is then subjected to a condensation reaction. This method is described in Japanese Patent Application Laid Open [Kokai or Unexamined] Numbers 50-77500 [77,500/1975] and 57-40526 [40,526/1982]).
Because both the cohydrolysis and condensation reactions in this preparative method cause an increase in the molecular weight of the silicone resin, the molecular weight of the final resin cannot be controlled. In addition, a highly reproducible production of silicone resin with the same molecular weight is beyond the capability of this preparative scheme due to the different reactivities of the D-D, D-T, and T-T bonds in a silicone resin composed of difunctional siloxane units (D unit) and trifunctional siloxane units (T unit). Moreover, the silicone resin afforded by this preparative scheme is itself associated with certain problems. For example, due to its large molecular weight dispersivity, it has a broad glass transistion point and softening point. This characteristic limits the scope of application of the silicone resin.
The present invention resulted from extensive efforts directed at solving the aforementioned problems associated with DT type silicone resins.
One objective of the present invention is to provide a method for producing silicone resin composed of difunctional siloxane units (D unit) and trifunctional siloxane units (T unit) the final resin exhibits a low molecular weight dispersivity and a controlled reproducible molecular weight.